Turbochargers are a type of forced induction system. They compress the air flowing into an engine, thus boosting the engine's horsepower without significantly increase its weight. Turbochargers use the exhaust flow from the engine to spin a turbine, which in turn spins an air compressor. Since the turbine spins about 30 times faster than most car engines and it is hooked up to the exhaust, the temperature in the turbine are very high.
One of the main problems with turbochargers is that they do not provide an immediate power boost when you step on the gas, so-called turbo lag or boost lag. One way to decrease turbo lag is to reduce the diameter of the turbine. However, a smaller turbine does not provide enough boost at high engine speeds. It is also in danger of spinning too quickly at higher engine speeds when lots of exhaust is passing through the turbine. A larger turbocharger can provide lots of boost at high engine speeds, but may have bad turbo lag because of how long it takes to accelerate its heavier turbine and compressor.
Twin-turbo systems have been designed to solve the above problem. A twin-turbo system can be a parallel twin-turbo system or a sequential twin-turbo system. A parallel twin-turbo system has two small turbos, one being active across the entire revolution range of the engine and the other becoming active at higher RPM. A sequential twin-turbo system, also called a two-stage turbocharging system, uses two turbochargers of different sizes. The smaller one spins up to speed very quickly, reducing lag, while the bigger one takes over at higher speeds to provide more boost.
Conventionally, the larger low pressure (LP) and smaller high pressure (HP) turbine housings of a two-stage turbocharging system are connected rigidly by, for example, a V-band connection and slip joint, in order to prevent exhaust gas leak. However, this kind of rigid connection could cause stresses under high temperature due to thermal expansion.
In some two-stage turbocharging systems, the smaller unit can be cantilevered off the larger unit so it is free to grow under thermal conditions. However, when both turbine units are sufficiently large, since this kind of connection does not provide reliable support, both turbine units have to be rigidly mounted to the engine, which means that under thermal growth, large stresses would be created if the mounting flanges were rigid.